Method for preparing biodegradable films from plant-based raw materials

ABSTRACT

There is provided a method for preparing biodegradable films from plant-based raw materials in the form of carbohydrates which method is continuous and carried out in one step. In the process the plant-based raw materials are modified and plastified and the films are prepared without interruption and in one step. The advantages of the invention reside in reduced shear stress of the starch molecules and thus in the improved properties of the films as well as in the cost-efficient, time-, energy- and space-saving manufacture.

FIELD OF THE INVENTION

The invention relates to a method for preparing biodegradable films fromplant-based raw materials. Such methods are gaining more and moreimportance, for example, in the production of environmentally compatiblepacking material. The object is to produce films having sufficientstrength that can easily be disintegrated or dissolved into theiroriginal natural components.

BACKGROUND OF THE INVENTION

Methods for preparing thermoplastic granules by using starches have beendisclosed previously (U.S. Pat. Nos. 4,133,784, 4,021,388, DE-A-23 22440). Such granules are processed to films in a second procedural step.In so doing, the starch is used as filler material in syntheticpolymers. Furthermore, methods for preparing and modifying destructuredthermoplastic starch have been disclosed (EP-A-378 646, EP-A-397 819).

WO 90/05161 A1 shows thermoplastically processable starch as well as amethod for preparing it. In the process aggregates and, if desired,various further additives are added to raw starch material to reduce themelting temperature of the starch to a level below its decompositiontemperature. This is to improve the structure of the starch melt so asto facilitate handling in preparing and processing thermoplastic starch.

EP-A1-0 474 095 discloses a process of preparing biodegradable articles,such as dishes, cups, cardboard boxes, etc. using plant-based rawmaterial, such as starch, as starting material. This material issupplemented with additives, plastified and converted in a firstextruder to an intermediate to form a billet or granules. Then theintermediate is molded in a second extruder into the desired article.

Similarly, conventional film preparation from plant-based raw materialsproceeds in two completely separate steps. The biopolymer meltdestructured and plastified in the extrusion process is cooled at theend of the first step, forced through a die under high pressure andgranulated. In order to avoid biopolymer foaming, it is required to coolthe melt. The resulting high shear stress causes damage to the molecularstructure of the starch while various molecule fragments are split off.The polymer chains of the starch are shortened and the molecular weightis reduced. The obtained starch granules are then processed to films ina further extrusion process subject to similar conditions. In theprocess the biopolymer is again decomposed as the molecular weight isreduced and the chains are shortened. The two-fold damage to themolecular structure, especially the shortening of the amylose chains,causes immediate deterioration of the properties (tensile strength,elasticity, etc.) of the thus obtained films. What is more, the two-foldextrusion is time-, energy- and cost-consuming.

By contrast, the object underlying the present invention is to provide acost-efficient method for preparing biodegradable films from plant-basedraw materials while avoiding the drawbacks of the prior art, improvingthe quality of the obtained films and achieving, for instance, hightensile strength and elasticity.

This object is achieved by providing the features indicated in theclaims.

SUMMARY OF THE INVENTION

In achieving the object, the invention is based on the concept ofoptionally disintegrating and modifying plant-based raw materials, e.g.,in the form of carbohydrates, in a continuous and one-step processwithout involving intermediates and without interruption, of plastifyingas well as of compressing and molding them into films. A suitableconfiguration of a screw-type extruder to transport the film materialsand suitable temperature conditions ensure careful processing of thebiopolymers.

The plant-based raw materials can include carbohydrates, e.g., flour ornative starch in natural or hybrid form-derived, for example, frompotatoes, manioc, peas, beans, corn, wax corn, corn with high amylosecontent, grain such as wheat and fractions prepared therefrom, barley orsorghum, starch derivatives consisting of physically and/or chemicallymodified starch, cellulose derivatives, plant rubber (carbohydratepolymer), hemicellulose, polysaccharides, hydrocolloids or mixtures ofone or more of those raw materials.

The invention has the following advantages.

The method of the invention reduces the shear strength, fragmentationand molecular weight loss of the biopolymers. As a result, thepreparation process is considerably facilitated because the reactionextruder and the film preparation form a process unit. The storage ofhygroscopic pellets in an aluminum compound under vacuum is no longernecessary. Compounding and film preparation occur in one step. Theproduction is more cost-efficient as no packing and transportation ofthe pellets are required and as less personnel and equipment costs areincurred. In addition, energy for transporting and melting the pelletscan be saved. The obtained films exhibit improved properties incomparison to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated below by means of drawings.

FIG. 1a shows a lateral view of a production line of the invention forthe production of blown films.

FIG. 1b shows a top view of a production line of the invention as shownin FIG. 1a.

FIG. 2a shows a lateral view of a production line of the invention forthe production of flat films.

FIG. 2b shows a top view of a production line of the invention as shownin FIG. 2a.

FIG. 3a shows an embodiment of the invention featuring the end of ascrew-type extruder comprising a double tubular die.

FIG. 3b shows an embodiment of the invention featuring the end of ascrew-type extruder comprising a tubular die.

FIG. 3c shows an embodiment of the invention featuring the end of ascrew-type extruder comprising a flat die.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1a and 1b show a production line of the invention for theproduction of blown films in two views. Depicted are individualproduction sections 1 to 6 and a die 7 interconnected by a screw-typeextruder (not shown) that transports and mixes or kneads the filmmaterials. A mixture of solid plant-based raw materials and, if desired,additives is measured with a metering device 17 (only shown in FIG. 1a)and added via the first inlet 11 of the first section 1. At the sametime, liquid additives can be added via the second inlet 12. Examples ofadditives include emulsifiers, plastifiers, preservatives and water. Insection 2 the mixture is heated and kneaded. Liquid additives can beadded via a third inlet 13. In a third section 3 the temperature isfurther increased and the plant-based raw materials are allowed to reactwith the additives, thus forming a melt. In a fourth section 4 the meltis evacuated in a vacuum apparatus 14 by allowing water to evaporatefrom the melt and thus causing the melt to cool down. The melt isfurther cooled in a fifth section 5. In a sixth section 6 the melt iscompressed so as to cause a pressure built-up in the die and then blownthrough a tubular die 7 to form a film 15. A similar production line isdepicted in FIGS. 2a and 2b (metering device 17 is only shown in FIG.2a). The difference between FIGS. 2a, 2b and FIGS. 1a, 1b lies in thefact that a flat film 16 is molded by a flat die 9.

FIG. 3a shows an embodiment of the invention wherein a blown film 15 isproduced by means of a double tubular die 8. Two one-lead screws 19 areprovided behind a double-lead screw 10 to transport the melt to the die8.

In FIG. 3b a blown film 15 is produced by means of a tubular die 7. Thedie is supplied with the melt by a gear pump 18 provided behind thedouble-lead screw 10.

In an embodiment comprising a flat die 9 for the production of a flatfilm 16 as shown in FIG. 3c the double-lead screw 10 feeds the meltdirectly to the die 9.

The screw-type extruder 10 is preferably shaped like a double-lead screwand preferably has the following configurations in each of sections 1 to6: right-handed in sections 1 and 2; right- and left-handed in section 3while optionally alternating several times; steeply right-handed insections 4 and 5; and flatly right-handed in section 6. These differingconfigurations ensure controlled and differing transport rates, kneadingand mixing in each of the sections and thus the desired careful handlingof the biopolymers.

Sections 1 to 6 and dies 7, 8 or 9 exhibit a controlled temperaturedistribution. Preferred temperature conditions include: 50° C. insection 1; 80°-100° C. in section 2; 120°-160° C. in section 3;120°-160° C. in section 4; 80° to 120° C. in section 5; 40°-60° C. insection 6; and 50° to 70° C. at dies 7, 8 or 9.

The embodiments of the method of the invention are shown below. Atwo-shaft screw-type extruder (ZSK 40) manufactured by Werner undPfleiderer, Germany, was used.

EXAMPLE 1

A solids mixture comprising potato starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder.

    ______________________________________                                        Solids:                                                                       Potato starch          99%                                                    Emulsifier             1%                                                                            100%                                                   Liquid additives:                                                             Water                  14%                                                    Glycerol               86%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 75%                                                    Liquids                25%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             80°                                                                            C.                                             Section 3:             120°                                                                           C.                                             Section 4:             120°                                                                           C.                                             Section 5:             80°                                                                            C.                                             Section 6:             60°                                                                            C.                                             Die:                   60°                                                                            C.                                             RPM:                   170     min.sup.-1                                     Torque:                30%                                                    Pressure:              80      bar                                            Blow ratio:            1:4                                                    ______________________________________                                    

A transparent film having the following properties was obtained:

    ______________________________________                                        Film thickness:        35     μm                                           Tensile strength along length:                                                                       12.1   N/mm.sup.2                                      Tensile strength along width:                                                                        11.8   N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              15     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 2

A solids mixture comprising potato starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder.

    ______________________________________                                        Solids:                                                                       Potato starch          99%                                                    Emulsifier             1%                                                                            100%                                                   Liquid additives:                                                             Water                  20%                                                    Glycerol               80%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 75%                                                    Liquids                25%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             80°                                                                            C.                                             Section 3:             140°                                                                           C.                                             Section 4:             140°                                                                           C.                                             Section 5:             100°                                                                           C.                                             Section 6:             60°                                                                            C.                                             Die:                   60°                                                                            C.                                             RPM:                   170     min.sup.-1                                     Torque:                20%                                                    Pressure:              60      bar                                            Blow ratio:            1:4                                                    ______________________________________                                    

A transparent film having the following properties was obtained:

    ______________________________________                                        Film thickness:        30     μm                                           Tensile strength along length:                                                                       10.2   N/mm.sup.2                                      Tensile strength along width:                                                                        9.4    N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              11     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 3

A solids mixture comprising potato starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder.

    ______________________________________                                        Solids:                                                                       Potato starch          99%                                                    Emulsifier             1%                                                                            100%                                                   Liquid additives:                                                             Water                  10%                                                    Glycerol               90%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 75%                                                    Liquids                25%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             80°                                                                            C.                                             Section 3:             160°                                                                           C.                                             Section 4:             160°                                                                           C.                                             Section 5:             120°                                                                           C.                                             Section 6:             60°                                                                            C.                                             Die:                   60°                                                                            C.                                             RPM:                   230     min.sup.-1                                     Torque:                15%                                                    Pressure:              52      bar                                            Blow ratio:            1:4                                                    ______________________________________                                    

A transparent film having the following properties was obtained:

    ______________________________________                                        Film thickness:        33     μm                                           Tensile strength along length:                                                                       9.4    N/mm.sup.2                                      Tensile strength along width:                                                                        8.2    N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              8      minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 4

A solids mixture comprising corn starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder.

    ______________________________________                                        Solids:                                                                       Corn starch            98%                                                    Emulsifier             2%                                                                            100%                                                   Liquid additives:                                                             Water                  14%                                                    Glycerol               86%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 70%                                                    Liquids                30%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             80°                                                                            C.                                             Section 3:             140°                                                                           C.                                             Section 4:             140°                                                                           C.                                             Section 5:             100°                                                                           C.                                             Section 6:             60°                                                                            C.                                             Die:                   60°                                                                            C.                                             RPM:                   200     min.sup.-1                                     Torque:                18%                                                    Pressure:              45      bar                                            Blow ratio:            1:3                                                    ______________________________________                                    

An opaque film having the following properties was obtained:

    ______________________________________                                        Film thickness:        42     μm                                           Tensile strength along length:                                                                       8.2    N/mm.sup.2                                      Tensile strength along width:                                                                        6.8    N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              24     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 5

A solids mixture comprising wheat flour and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder.

    ______________________________________                                        Solids:                                                                       Type 405 wheat flour   98%                                                    Emulsifier             2%                                                                            100%                                                   Liquid additives:                                                             Water                  25%                                                    Glycerol               75%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 70%                                                    Liquids                30%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             80°                                                                            C.                                             Section 3:             140°                                                                           C.                                             Section 4:             140°                                                                           C.                                             Section 5:             100°                                                                           C.                                             Section 6:             60°                                                                            C.                                             Die:                   60°                                                                            C.                                             RPM:                   200     min.sup.-1                                     Torque:                20%                                                    Pressure:              55      bar                                            ______________________________________                                    

The material was molded by a flat slot die and drawn out to a film.

An opaque film having the following properties was obtained:

    ______________________________________                                        Film thickness:        94     μm                                           Tensile strength along length:                                                                       9.6    N/mm.sup.2                                      Tensile strength along width:                                                                        9.0    N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              64     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 6

A solids mixture comprising potato starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder. At the endof the two-shaft section the material was fed to a tubular die via agear pump.

    ______________________________________                                        Solids:                                                                       Potato starch          99%                                                    Emulsifier             1%                                                                            100%                                                   Liquid additives:                                                             Water                  14%                                                    Glycerol               86%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 80%                                                    Liquids                20%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             100°                                                                           C.                                             Section 3:             140°                                                                           C.                                             Section 4:             140°                                                                           C.                                             Section 5:             80°                                                                            C.                                             Section 6:             40°                                                                            C.                                             Die:                   50°                                                                            C.                                             RPM:                   200     min.sup.-1                                     Torque:                62%                                                    Pressure at pump inlet:                                                                              80      bar                                            Pressure at pump outlet:                                                                             210     bar                                            Blow ratio:            1:4                                                    ______________________________________                                    

A transparent film having the following properties was obtained:

    ______________________________________                                        Film thickness:        29     μm                                           Tensile strength along length:                                                                       14.1   N/mm.sup.2                                      Tensile strength along width:                                                                        12.6   N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              10     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

EXAMPLE 7

A solids mixture comprising potato starch and an emulsifier(mono-diglyceride) as well as the liquid components water and glycerolwere continuously added to the two-shaft screw-type extruder. At the endof the two-shaft section the material was passed on to two one-leadscrews and from there on fed to a double tubular die 8. In the screwstwo concentric tubes were formed from the two product flows, the thusobtained tubes were immediately laminated one on top of the other andtogether blown to a film.

    ______________________________________                                        Solids:                                                                       Potato starch          99%                                                    Emulsifier             1%                                                                            100%                                                   Liquid additives:                                                             Water                  20%                                                    Glycerol               80%                                                                           100%                                                   Mixing ratio during the process:                                              Solids                 75%                                                    Liquids                25%                                                    Temperature profile:                                                          Section 1:             50°                                                                            C.                                             Section 2:             100°                                                                           C.                                             Section 3:             120°                                                                           C.                                             Section 4:             120°                                                                           C.                                             Section 5:             80°                                                                            C.                                             Section 6:             60°                                                                            C.                                             Die:                   70°                                                                            C.                                             RPM:                   160     min.sup.-1                                     Torque:                44%                                                    Pressure:              50      bar                                            Blow ratio:            1:4                                                    ______________________________________                                    

A transparent film having the following properties was obtained:

    ______________________________________                                        Film thickness:        38     μm                                           Tensile strength along length:                                                                       11.4   N/mm.sup.2                                      Tensile strength along width:                                                                        9.4    N/mm.sup.2                                      Duration of dissolution of 10 g of film                                                              31     minutes                                         in 100 g of water at 20° C.:                                           ______________________________________                                    

We claim:
 1. A method for preparing biodegradable films from plant-basedraw materials, preferably in the form of carbohydrates by means of ascrew-type extruder, wherein modification or disintegration and theplastification of the plant-based raw materials and subsequent filmpreparation are done continuously and in one step comprising the stagesof:a. continuously adding measured plant-based raw materials and, ifdesired, solid additives through a first inlet (11) as well as, ifdesired, liquid additives through a second inlet (12) in a firstextruder section (1) and mixing them to form a mixture, b. heating andkneading the mixture in a second extruder section (2), c. heating andreacting the mixture in a third extruder section (3) to form a melt, d.evacuating said melt in a vacuum apparatus (14) in a fourth extrudersection (4) by extracting and allowing water to evaporate from the melt,thus causing the melt to cool down, e. further cooling the melt in afifth extruder section (5), f. compressing the melt in a sixth extrudersection (6), g. extruding the melt from the extruder through a die (7,8, 9) to form a film (15, 16).
 2. A method as defined in claim 1 whereinin stage b. liquid additives are added through a third inlet (13).
 3. Amethod as defined in claim 1 wherein the plant-based raw materials isselected from the group consisting of flour or native starch in naturalor hybrid form derived from potatoes, manioc, peas, beans, corn, waxcorn, corn with high amylose content, grain selected from wheat andfractions prepared therefrom, barley or sorghum, starch derivativesconsisting of physically or chemically modified starch, cellulosederivatives, plant rubber, hemicellulose, polysaccharides, hydrocolloidsor mixtures of one or more of those raw materials.
 4. A method asdefined in claim 1 wherein the temperature is selectively adjustable inthe first to sixth sections (1 to 6).
 5. A method as defined in claim 1wherein the die is a tubular die (7) and a blown film (15) is molded. 6.A method as defined in claim 1 wherein the die is a double tubular die(8) and a blown film (15) is molded.
 7. A method as defined in claim 1wherein the die is a flat slot die (9) and a flat film (16) is molded.8. A method as defined in claim 1 wherein a metering device (17) for theplant-based raw materials and additives is provided in front of thefirst inlet (11).
 9. A method as defined claim 1 wherein a gear pump(18) or at least one one-lead screw (19) is provided between thescrew-type extruder (10) and the die (7, 8, 9).
 10. A method as definedin claim 1 wherein the screw-type extruder (10) is a two-shaftscrew-type extruder.
 11. A method as defined in claim 10 wherein thetwo-shaft screw-type extruder (10) has the following configuration:a.right-handed with a first slope in the first and second sections (1 and2, respectively), b. right- and left-handed with the first slope in thethird section (3) while alternating one or several times. c. steeplyright-handed with a second slope in the fourth and fifth sections (4 and5, respectively), and d. flatly right-handed with a third slope in thesixth section (6), wherein the second and the third slopes are greaterand smaller, respectively, than the first slope.
 12. A method as definedin claim 4 wherein the following temperature conditions are adhered to:10° to 80° C., preferably 50° C., in the first section (1); 10° to 150°C., preferably 80° to 100° C., in the second section (2); 70° to 200°C., preferably 120° to 160° C., in the third section (3); 70° to 200°C., preferably 120° to 160° C., in the fourth section (4); 70° to 200°C., preferably 80° to 120° C., in the fifth section (5); 40° to 200° C.,preferably 40° to 60° C., in the sixth section (6); 40° to 150° C.,preferably 50° to 70° C., at the die (7; 8; 9).
 13. A method as definedin claim 1 wherein the films in the dry state contain 40 to 100% by wt.,preferably 75 to 95% by wt., of carbohydrates.
 14. A method as definedin claim 1 wherein glycerol-monostearate is added as an additivepreferably in the first section (1) to act as an emulsifier.
 15. Amethod as defined in claim 1 wherein glycerol is added as an additivepreferably in the second section (2) to act as a plastifier.
 16. Amethod as defined in claim 1 wherein water is added as an additivepreferably in the second section (2) to act as a solvent.
 17. A methodas defined in claim 4 wherein potassium sorbate is added as an additivepreferably in the first section (1) to act as a preservative.
 18. Amethod as defined in claim 1 wherein the films (15; 16) in the dry statecontain 0 to 25% by wt., preferably 0.1 to 2.5% by wt., of glycerolmonostearate, 0 to 40% by wt., preferably 10 to 28% by wt., of glycerol;and 0 to 5% by wt., preferably 0.1 to 0.5% by wt., of potassium sorbate.19. A method as defined in claim 1 wherein the films (15; 16) contain 0to 35% by wt., preferably 10 to 20% by wt., of water.
 20. Films fromplant-based raw materials producible by the method as defined in claim1.